The first geothermal project in Switzerland designed to generate electricity was launched in Basel. In December 2006, after an exploratory phase, water was due to be forced under high pressure into the crystalline basement rock over a two-week period. The aim of this procedure was to increase its permeability at a depth of between 4,000 and 5,000 m and create a geothermal reservoir where the liquid would circulate and heat up.
The geothermal injection process was tracked by a dense network of seismic monitoring stations, including six of the borehole seismometers operated by Geopower Basel AG at depths of between 300 and 2,700 m. As anticipated, these stations recorded thousands of microquakes. The SED runs a compact network of seismic stations in the Basel area, which was substantially densified during the Deep Heat Mining Project. In addition to this, the federal state of Baden-Württemberg's Earthquake Office (LED) set up a number of monitoring stations on the German side of the border with Switzerland. The SED had access to the data generated by these stations and was tasked to estimate the magnitude of the detected earthquakes.
The rate of injection (quantity of liquid over time) was gradually increased until, on the sixth day, the maximum rate was reached. Shortly after, an earthquake with a local magnitude of 2.6 occurred, whereupon the rate of injection was first decreased and then stopped altogether a few hours later. Approximately five hours after that, an earthquake with a local magnitude of 3.4 (moment magnitude 3.1) occurred. Having an intensity of V ("moderate" on the Mercalli scale), it was felt over a wide area and caused minor damage. Three more quakes with local magnitudes of higher than 3.0 ensued, the last occurring in February 2007. All in all, more than 200 earthquakes with magnitudes of 0.9 (ML) or above were recorded (equivalent to over 900 earthquakes with Mw). Consequently, the project managers stopped implementing their plan and definitively terminated the project in 2009 after a comprehensive risk assessment.
For the most part, the damage reported concerned small cracks in buildings' plasterwork. Most claims, which totalled CHF 6 million, resulted in the payment of compensation.